One of the more prevalent types of freezers used to provide cryogenic freezing of a product (e.g. foodstuffs) is a continuous, in-line tunnel that utilizes liquid nitrogen as an expendable refrigerant. One such apparatus in commercial use is shown in U.S. Pat. No. 3,813,895 and U.S. Pat. No. 3,892,104, the specifications of both patents being incorporated herein by reference. The apparatus of the prior art can achieve high thermal efficiency because it is designed as a counterflow heat exchanger. The product moves through the tunnel on a continuous belt from an entry end (portal or opening) to a discharge end (portal or opening). Liquid nitrogen is sprayed onto the food product at a location adjacent to the discharge end (opening) of the freezer. The cold nitrogen gas, at -320.degree. F. (-196.degree. C.), evolved in the liquid nitrogen spray zone, moves through multiple zones of gas recirculation as it flows toward the entrance of the freezer. Since the maximum available refrigeration has been utilized at that point, the warmed nitrogen gas can then be vented to the outside atmosphere by an exhaust system placed proximate the entry end of the tunnel.
Liquid nitrogen that is in equilibrium at 35.0 psia (241 kpa) has a latent heat of 80.5 BTU/lb. (187 J/g) when vaporized at atmospheric pressure. When the product enters the freezer at 75.degree. F. (24.degree. C.), the nitrogen gas will leave the freezer entrance at approximately 0.degree. F. (-18.degree. C.) in a freezer such as shown in the aforementioned patents and offered for sale by Air Products and Chemicals, Inc. as a CRYO-QUICK.RTM. freezer. At these conditions the freezer is operating at optimum thermal efficiency and the nitrogen gas will have a sensible heat of 79.5 BTU/lb. (185 J/g). Thus, the liquid nitrogen has a total available refrigeration of 160 BTU/lb. (372 J/g). Since the sensible heat of the nitrogen gas is almost one-half of the total available refrigeration, it is necessary to provide correct nitrogen gas flow through the freezer to achieve high thermal efficiency.
The amount of liquid nitrogen injected into the freezer will depend upon the amount of refrigeration required by the product to be frozen (e.g. foodstuff). Further, whenever production is interrupted, the liquid nitrogen flow rate should be reduced substantially to maintain the freezer at its operating temperature. In a typical CRYO-QUICK freezer, having a conveyor belt of 28" (711 mm) width and a length of 66' (20 m), the liquid nitrogen flow rate will vary from 3065 to 358 lb/hr (1390 to 162 kg/hr). In addition, the most efficient operation is obtained when the liquid nitrogen flow is shut off completely during the production interruption. If the production is stopped for a long period of time, then liquid nitrogen is readmitted to the freezer based upon the temperature within the freezer. Thus, the nitrogen gas flow through the freezer must change over a wide range from the maximum flow to zero flow.
If the gas flow control system moves a larger volume of gas than the amount of gaseous nitrogen evolved in the liquid nitrogen spray zone, warm room air will be pulled into the discharge opening of the freezer. The entry of warm room air will be a significant heat input, causing a loss of thermal efficiency. Further, the moisture contained in the room air will result in frost and ice accumulation within the freezer and impair its performance. If the gas flow control system moves a smaller volume than required, cold nitrogen gas will spill out of the discharge opening, causing a significant loss in thermal efficiency. Also, the nitrogen gas spilling into the processing room can cause an oxygen deficient condition that could result in a serious safety hazard.
In early freezers represented by U.S. Pat. No. 3,345,828, to insure that the cold gas would flow countercurrent to the product flow, parallel fans were employed in the tunnel. A thermocouple placed at the collection point of cold gas, where it interfaces with warm gas, was used to detect the level of the hot/cold interface and to change position of a damper (76) to equalize volume of circulation between the parallel flow fans. While this method proved satisfactory for freezers employing parallel flow fans, patentees in U.S. Pat. No. 3,403,527 improved this apparatus by employing additional dampers with the parallel flow fans.
Subsequent to the early parallel flow fan type freezers, it was discovered that a radial flow fan could be used to force the gas in countercurrent flow to the product. U.S. Pat. No. 3,813,895 discloses the type of freezer using all radial fans wherein a curved damper, which is temperature actuated, can be used to control the total flow of gas in the freezer. However, it was found that this apparatus performed satisfactorily on freezers of small dimensions (e.g. tunnel length of 22 ft. or less). The patentees in U.S. Pat. No. 3,892,104 employed a centrifugal fan to move the cold cryogen toward the entry end of the tunnel. Control of the fan and hence control of the movement of gas through the tunnel was effected by sensing the spray header pressure which in turn controlled the speed of the fan.
U.S. Pat. No. 4,528,819 discloses an immersion-type cryogenic freezer suitable for freezing foodstuffs wherein movement of the vaporized cryogen is in concurrent flow with the movement of the product through the freezer. Patentees disclose control of an exhaust fan to control the direction of vaporized nitrogen flow, which in turn prevents air insufflation into the freezer. However, an exhaust fan cannot be used effectively in a tunnel type freezer to move the vaporized cryogen through the freezer. When the freezer is more than 30 ft long, the exhaust fan is unable to move a sufficient volume of vaporized cryogen through the freezer. Although an exhaust fan could be used on smaller freezers, the exhaust fan will also pull room air through the entry end opening of the freezer. When moist room air is mixed with the vaporized cryogen, the moisture will become frost that will clog the exhaust duct. This condition is most severe when the vaporized cryogen is colder than -50.degree. F. and the relative humidity of the room air is greater than 50%.
A conventional CRYO-QUICK freezer with a control system according to that shown in U.S. Pat. No. 4,800,728 employs a constant speed exhaust blower that is selected for a capacity at least one and one-half times the volume of nitrogen gas to assure safe operation. However, when the freezer is operated within a refrigerated room to freeze a cool product, such as a hamburger patty at 32.degree. F. (0.degree. C.), a constant speed exhaust blower is not satisfactory. When processing a cool product, the entrance temperature of the freezer becomes substantially colder, i.e. -50.degree. F. (-46.degree. C.). The excess capacity of the exhaust blower (fan) draws a large volume of room air into the entrance opening of the freezer. As the room air enters the entrance opening, it impinges on the conveyor belt, warming the conveyor belt and increasing the heat loss into the freezer. Further, the warm, moist room air impinging on the cold -50.degree. F. (-46.degree. C.) conveyor belt deposits a layer of frost on the woven wire belt. Over a period of time, the frost layer thickens to restrict the openings in the conveyor belt. When this occurs, the recirculated nitrogen gas cannot pass through and under the conveyor belt. As a result, the bottom surface of the food product will not be adequately frozen.
Another problem with a constant speed exhaust blower is that warm, moist room air is mixed with cold nitrogen in the exhaust duct. When the freezer entrance temperature is -50.degree. F. (-46.degree. C.) or colder, the moisture forms frost that tends to accumulate within the exhaust duct. As the exhaust duct becomes clogged with frost, the flow through the exhaust system is restricted causing a potentially hazardous situation.
When using a constant speed exhaust blower another problem arises in regard to removal of refrigerated air from the processing room. Warm make-up air must enter the processing room to offset this loss, thereby significantly increasing the amount of mechanical refrigeration required to maintain the room at temperature, i.e. +50.degree. F. (10.degree. C.).
When the freezer is cold but not producing frozen food, such as during a lunch break, the LIN flow to the freezer is reduced to about 15% to maintain the freezer at operating temperature. Under those conditions, a constant speed exhaust blower tends to pull additional room air into the discharge opening of the freezer. The warm, moist air entering the discharge opening of the freezer increases the heat losses of the freezer. Further, the moisture forms frost that clogs the freezer, further impeding satisfactory performance.
For those reasons, it is desirable to provide an exhaust system with variable volume that is automatically adjusted to remove only nitrogen gas from the freezer with a minimum of room air.
The known solution to the problem of providing a variable volume exhaust blower employs a pressure transducer to detect the amount of LIN entering the freezer by sensing the pressure in the LIN spray header. In the first version of this system, the pressure transducer provided the speed signal to a DC power supply that varied the speed of a DC motor driving the exhaust blower. In the present system, the pressure transducer provides a speed signal to an AC inverter that varies the speed of an AC motor driving the exhaust blower. Although this system can perform satisfactorily during continuous production, it has several disadvantages. The nitrogen gas is delivered to the entrance of the freezer by a temperature activated gas flow control, e.g. U.S. Pat. No. 4,800,728, that operates independently of the LIN spray header pressure. Thus, during a process upset, the LIN spray pressure may change suddenly without a corresponding change in the gas flow fan speed. Consequently, the exhaust blower may slow down while the gas flow fan is still delivering a large volume of nitrogen gas to the freezer entrance.
Another disadvantage of this system is that it requires a LIN spray header pressure that is high enough to produce the required exhaust blower speed. Since the pressure transducer in the present system has a range of 0 to 10 psi (0 to 69 kPa), the LIN spray header pressure must be 10 psi (69 kPa) to operate the exhaust blower at full speed. In those cases where the LIN spray header pressure is 5 psi (34 kPa) or less, the exhaust blower may not operate at sufficient speed to remove all the nitrogen gas delivered to the freezer entrance.
Another disadvantage of this system is the fact that the mass flow through the LIN spray header is not constant with constant spray header pressure. If the equilibrium condition of the liquid nitrogen, as indicated by the LIN storage tank pressure, changes significantly, the quality of the LIN flowing through the spray nozzles will also change. For that reason, the LIN spray header pressure will be different for the same mass flow of liquid nitrogen. This same condition will occur if one or more of the LIN spray nozzles becomes clogged with debris. When either of these situations occur, the freezer operator must readjust the system to obtain the proper exhaust blower speed.